High speed robotic weighing system

Abstract

This disclosure pertains to weighing a physical item while it is moving in a servo-driven conveyor system for e-commerce, logistics, manufacturing and other applications. The introduction of an unknown mass to an electro-mechanical feedback or filter network controlling a conveyance system will modify the steady state behavior of that system in such a way that measuring the phase or frequency shift of an input signal or oscillation will enable us to infer the magnitude of that mass.

The invention claimed is: 
     
       1. A system for weighing items while they are moving, the system comprising:
 a scale conveyor arranged to receive at least one item from an infeed conveyor, the scale conveyor including a drive axle; 
 a servo motor having a shaft; 
 a servo drive system coupled to the server motor to drive the motor responsive to a command velocity input signal; 
 a motor mount arranged for coupling the servo motor shaft to the drive axle of the scale conveyor, the motor mount comprising a viscous rotary damper, a rotary spring and a shaft coupler, arranged to connect the servo motor shaft to the drive axle applying the rotary damper viscosity; 
 a velocity encoder coupled to the scale conveyor to generate velocity signals responsive to a current velocity of the scale conveyor; and 
 a processor configured to— 
 monitor at least one of (a) a frequency of oscillation and (b) a phase response of the velocity signals to detect a variation; 
 responsive to detecting the variation, analyze the variation of frequency of oscillation and or the phase response; and 
 infer a mass of the item(s) received on the scale conveyor based on the analysis. 
 
     
     
       2. The system of  claim 1  wherein:
 a frequency of oscillation before the item is received on the conveyor is a natural resonant frequency arising based on the velocity signals from the velocity encoder being fed back to the servo drive system as the command velocity input of the servo drive system so as to provide a positive feedback loop supporting natural self-exciting oscillation; and 
 the variation is detected by comparing the frequency of oscillation before the item is received on the conveyor to the frequency of oscillation after the item is received on the conveyor. 
 
     
     
       3. The system of  claim 1  wherein the phase response is determined by a phase difference between the servo drive system input command signal and the velocity signals generated by the velocity encoder. 
     
     
       4. The system of  claim 1  wherein the command velocity input signal comprises a time varying command velocity input signal. 
     
     
       5. The system of  claim 1  wherein the time varying command velocity input signal is a substantially sinusoidal signal. 
     
     
       6. The system of  claim 5  wherein the time varying command velocity input signal has a frequency on the order of 10 Hz. 
     
     
       7. The system of  claim 1  including an isolation roller disposed in between the infeed conveyor and the scale conveyor, the isolation roller positioned so as to isolate the item as it moves from the infeed conveyor to the scale conveyor so that the item rests on only one of the infeed conveyor and the scale conveyor at a given time. 
     
     
       8. The system of  claim 7  wherein the scale conveyor has a surface that defines a scale deck height that is parallel to a top surface of the isolation roller. 
     
     
       9. The system of  claim 7  and further comprising a second isolation roller disposed in between the scale conveyor and outfeed conveyor, the second isolation roller positioned so as to isolate an item moving from the scale conveyor to the outfeed conveyor so that the item is supported by only one of the scale conveyor and the outfeed conveyor at a given time. 
     
     
       10. A method of weighing an item comprising the steps of:
 providing a mechanical conveyor having a drive shaft for moving the item; 
 providing a servo motor having a motor shaft for driving the conveyor; 
 coupling the servo motor shaft to the conveyor drive shaft by means of a viscous damper; 
 providing a servo system for driving the servo motor, and arranging the servo system to drive the motor responsive to a time-varying velocity command input signal; 
 providing a motor shaft encoder coupled to the servo motor shaft to generate motor velocity signals; 
 providing a velocity encoder coupled to the conveyor to generate conveyor velocity signals; 
 driving the conveyor responsive to the time-varying velocity command input signal; 
 receiving first item onto the moving conveyor; and 
 determining a mass of the first item based on a phase angle between the motor velocity signals and the conveyor velocity signals. 
 
     
     
       11. The method of  claim 10  further comprising:
 receiving a second item onto the moving conveyor while the first item is still on the moving conveyor; 
 after receiving the second item, measuring a phase angle between the motor velocity signals and the conveyor velocity signals; and 
 determining a mass of the second item based on the measured phase angle. 
 
     
     
       12. The method of  claim 11  wherein the second item overlaps the first item in location along the conveyor. 
     
     
       13. The method of  claim 11  including:
 inputting a substantially sinusoidal signal as the time-varying velocity command input signal, the sinusoidal signal having a frequency F on the order of 10 Hz. 
 
     
     
       14. The method of  claim 10  including:
 estimating an impedance of the viscous damper based on the time-varying velocity command input signal; and 
 calculating the mass of the first item based on the phase angle being proportional to an inverse tangent function of a ratio of the viscous damper impedance to an impedance Xm of the item mass. 
 
     
     
       15. The method of  claim 11  including:
 calculating the item mass based on a relationship Xd/2πXm=F drive, wherein Xd is an impedance of the viscous damper at the frequency F drive. 
 
     
     
       16. The method of  claim 15  including:
 selecting a desired range of mass for weighing items; and 
 selecting Xd and F drive to produce a 45-degree phase shift at a midpoint of the desired mass range. 
 
     
     
       17. The method of  claim 16  wherein the desired range is approximately 0 to 100 pounds, thereby producing 45 degrees of phase angle between the motor velocity signals and the conveyor velocity signals when an item of 50 pounds mass is present. 
     
     
       18. A method of weighing a moving item comprising the steps of:
 providing a mechanical conveyor having a drive shaft for moving an item; 
 providing a servo motor having a motor shaft for driving the conveyor; 
 coupling the servo motor shaft to the conveyor drive shaft by means of a viscous damper and a rotary spring; 
 providing a servo system for driving the servo motor, and arranging the servo system to drive the motor responsive to a velocity command input signal, the input signal selected to drive the motor at a predetermined nominal constant velocity; 
 acquiring conveyor velocity signals responsive to motion of the conveyor; 
 superimposing the conveyor velocity signals on to the velocity command input signal to so as to form a positive feedback loop that supports natural self-exciting oscillation, the feedback loop including a feedback network comprising the viscous damper, the rotary spring, and the conveyor, so that the feedback loop oscillates at a natural resonant frequency of the feedback network while the conveyor is unloaded; 
 measuring the resonant frequency while the conveyor is unloaded; 
 receiving a first item onto the moving conveyor; and 
 determining a mass of the first item based on measuring a change in the resonant frequency relative to the resonant frequency measured while the conveyor was unloaded. 
 
     
     
       19. The method of  claim 18  including:
 storing calibration data that associates oscillation frequency to mass of an item; and 
 determining the mass of the first item based on the calibration data. 
 
     
     
       20. The method of  claim 18  including:
 estimating the natural resonant frequency by f=1/(2π√ (LC)) where L is the analog of the variable mass of the first item and C is the analog of the inverse of the rotary spring constant; and 
 selecting the rotary spring constant to produce a desired natural resonant frequency on the order of 10 Hz. 
 
     
     
       21. The method of  claim 18  including:
 receiving a second item onto the moving conveyor while the first item is still on the moving conveyor; and 
 determining a mass of the second item based on measuring a change in the resonant frequency relative to the resonant frequency measured before the second item arrived on the conveyor. 
 
     
     
       22. The method of  claim 21  wherein the second item overlaps the first item in location along the conveyor. 
     
     
       23. The method of  claim 18  including:
 receiving a stream of additional items onto the conveyor, and determining a mass per unit time of the additional items based on measuring changes in the resonant frequency relative to the resonant frequency measured before the additional items began arriving on the conveyor.